1. Field of Invention
The invention generally relates to medical apparatus and methods, and more specifically to prosthetic devices such as stents for use in the coronary and peripheral arteries as well as in other vessels and body lumens.
Stenting is an important treatment for patients with vascular occlusive disease. A stent is often a hollow, tubular implant designed to provide scaffolding to the walls of a diseased blood vessel or other body lumen in order to maintain patency of the vessel or lumen. Stents are usually placed at the site of a stenotic lesion within an artery after a primary treatment such as angioplasty. They are typically delivered to the arteries using long, flexible catheters inserted percutaneously through the patient's femoral artery.
Stents are typically either balloon expandable or self-expandable. Balloon expandable stents are mounted over a balloon or other expansion element on a delivery catheter. When the balloon is inflated, the balloon expands and correspondingly expands and deforms the stent to a desired diameter. The balloon can then be deflated and removed, leaving the stent in place.
A self-expanding stent is simply released from the delivery catheter so that it expands until it engages the vessel wall. Self-expanding stents are typically delivered to a treatment site while compressed or crimped within a constraining sheath. Retraction of the sheath removes the constraint and allows the stent to radially expand into engagement with the vessel wall.
Self-expanding stents may be made of materials having high elasticity or a shape memory alloy such as Nitinol, a nickel-titanium (NiTi) alloy. Resilient, self-expanding stents are often used in the peripheral vascular system and regions of the body closer to the body's surface because their resilience helps minimize damage or crushing caused by body movement or externally applied forces.
Self-expanding stents may also have certain advantages over balloon expandable stents in the treatment of arteries. Balloon expandable stents are often expanded to a constant diameter and do not conform well with an artery having variations in diameter due to vessel tortuosity or taper. Because of the potential mismatch between stent diameter and vessel diameter, there is a possibility for gaps to form between the outer stent surface and the inner vessel wall, and this can lead to thrombosis formation. Self-expanding stents expand until their outer surface is constrained by contact with the vessel wall and thus the use of a self-expanding stent may eliminate or reduce these gaps thereby reducing thrombosis formation.
Another trend in stenting is the use of longer stents for treatment of long diffuse lesions in the peripheral vessels. While this procedure is promising, some challenges still exist. For example, longer stents are often less flexible and therefore are harder to deliver and deploy in torturous vessels, and they can fracture or kink. Therefore, there is a need for longer stents with improved flexibility.
The flexibility of stents may be improved through the variation of the stent geometry. Current stents are typically articulated tubes consisting of a series of axially adjacent, tubular rings interconnected with one another by one or more bridges. Each tubular ring often consists of several repeating cells formed from axially oriented struts, each having the same length. The cells are often arranged to form a helical, zig-zag, diamond, rectangular, undulating, mesh, or other pattern. Using struts of the same length in a repeating cell pattern results in uniform stent compression and expansion thereby helping to ensure uniform loading of a stent with a delivery catheter as well as uniform scaffolding of the treatment site after the stent has been deployed. The tubular rings are often rigid, providing mechanical support to the vessel but little or no axial flexibility. Thus, axial flexibility of the stent may be adjusted by using bridges to connect adjacent tubular rings. While it is desirable to keep the tubular rings close together in order to provide maximum scaffolding to the vessel, it is also desirable to have a longer gap between the tubular rings so that a longer bridge may connect adjacent tubular rings for greater stent flexibility. Additionally, by bringing tubular rings closer together or interleaving segment ends, the overall stent column strength is increased along with the stent's ability to resist twisting. This allows the stent rings to be coupled together with fewer bridges, thereby also increasing stent flexibility. Stent column strength is important during retraction of a constraining sheath during stent deployment. Friction between the sheath and the tubular rings may twist them relative to one another or buckle the stent forcing the segments together, thereby causing binding or potentially interfering with proper stent deployment. Thus, it is desirable to provide an improved stent that provides improved flexibility without substantially diminishing scaffolding ability or column strength, as well as providing other structural advantages while avoiding some of the aforementioned challenges.
2. Description of the Background Art
Prior patents and publications describing various stent geometries and stent delivery systems include U.S. Pat. Nos. 5,421,955; 5,716,393; 6,022,374; 6,132,460; 6,264,688; 6,273,911; 6,334,871; 6,375,676; 6,464,720; 6,582,460; 6,918,928; U.S. Patent Publication No. 2003/0114919; International PCT Publication WO 2008/005111; and European Patent No. EP 1318765.